CI: add docker-build stage

.gitlab-ci.yml

@@ -53,3 +53,18 @@ go-build:
   artifacts:
     paths:
       - bin/
+
+docker-build:
+  image: docker:20
+  stage: build
+  services:
+    - docker:20-dind
+  variables:
+    IMAGE_TAG: $CI_REGISTRY_IMAGE/ubsserver:$CI_COMMIT_REF_SLUG
+    DOCKER_TLS_CERTDIR: "/certs"
+  before_script:
+    - docker version
+  script:
+    - docker login -u $CI_REGISTRY_USER -p $CI_REGISTRY_PASSWORD $CI_REGISTRY
+    - docker build -f ./docker/ubsserver/Dockerfile -t $IMAGE_TAG .
+    - docker push $IMAGE_TAG